E CEE 120-Index prop and grain size - Index and...

Index and classification properties of soils Index properties of soils – phase relations Soil texture and grain size Grain size distribution and particle shape 1.3 Index properties of soils Soil is a three-phase material and its behavior depends on the interaction of its three phases. Any mass of soil consists of a collection of solid particles with voids in between which can be filled either with water, air, or filled partly with both water and air. Consequently, the total volume, V t , of the soil mass consists of the volume of soil solids, V s , and the volume of voids, V v . The volume of voids is made up of the volume of water, V w , and the volume of air, V a . We can schematically represent these three phases in a phase diagram shown in Fig. 34, in which each of the three phases is shown separately. On the left side are the volumes of the three phases, while on the right side are the corresponding masses of the phases. This diagram is depiction of a prism the base area of which is one. In engineering practice, we usually measure the total volume, V, the mass of water, M w , and the mass of dry solids, M s . Then we calculate the rest of the values and the mass-volume relationships that we need. Most of these relationships are independent of sample size, and they are often dimensionless. There are three volumetric ratios that are very useful in geotechnical engineering, and these can be determined directly from the phase diagram in Fig. 34. The void ratio, e, is defined as s v V V solids of volume voids of volume e = = . Eq. 1.4 1

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The void ratio, e , is normally expressed as a decimal. The typical values of void ratios for sands may range from 0.4 to about 1.0, while typical values for clays vary from 0.3 to 1.5 and even higher for some organic soils that have very open structure. The porosity, n, is defined as (%) 100 (%) 100 x V V x volume total voids of volume n t v = = . Eq. 1.5 Porosity is usually expressed in percentages. Void ratio and porosity are related in the following manner: e e n + = 1 and n n e − = 1 Eq. 1.6 The degree of saturation tells us what percentage of the total volume of voids contains water: (%) 100 (%) 100 x V V x voids of volum watewr of volume S v w = = . Eq. 1.7 If the soil is completely dry, then S = 0% , and if the pores are completely full of water, then the soil is fully saturated and S = 100%. There is just one mass ratio used frequently in soil mechanics, water content (sometimes called moisture content), but this property is about the most important property of soil: (%) 100 (%) 100 x M M x solids of mass water of mass w s w = = . Eq. 1.8 Water content can range from zero (dry soil) to several hundred percent, up to 500%. The natural water content for most soils is, however, well under 100%. Other useful ratios in geotechnical engineering, as well as in many other engineering disciplines,

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